Method and Apparatus for Sensing Downhole Characteristics

ABSTRACT

A system for sensing a borehole or formation characteristic at a selected depth while a drill string  12  is in the borehole  36  includes a plurality of sensors  40  to positioned at axially spaced locations along the drill string  12 , and a data transmission system  46  for transmitting signals from each of the plurality of sensors to the surface. A surface computer  22  receives the transmitted signals and determining the sensed characteristics at a selected depth as a function of one of a plurality of sensors positioned along the drill string while at a first selected depth and subsequently another of the plurality of sensors is along the drill string while at substantially the first selected depth.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a non-provisional application claiming priority from U.S. provisional application Ser. No. 60/804,015, filed on Jun. 6, 2006—the entire disclosure of which is incorporated by reference herein for all it contains.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to well drilling operations and, more particularly, to sensing a change in downhole conditions over time and depth while a drill string is in the well.

2. Description of the Related Art

Drilling operators logically need as much information as possible about borehole and formation characteristics while drilling a well for safety and reserves calculations. If problems arise while drilling, minor interruptions may be expensive to overcome and, in some cases, pose a safety risk. Since current economic conditions provide little margin for error and cost, drilling operators have a strong incentive to fully understand downhole characteristics and avoid interruptions.

Gathering information from downhole can be challenging, particularly since the downhole environment is harsh, ever changing, and any downhole sensing system is subject to high temperature, shock, and vibration. In many wells, the depth of the well at which the sensors are positioned causes significant attenuation in the signals which are transmitted to the surface. If signals are lost or data becomes corrupted during transmission, the operator's reliance on that data may result in significant problems. Accordingly, many downhole conditions sensed while drilling a well have reliability concerns.

Typically, various types of sensors may be placed at a selected location along the bottom end of the drill string, and a mud pulser, which is part of a measurement-while-drilling (MWD) system is widely used in the oilfield industry to transmit and send signals to the surface. Signals from bottom hole sensors may be transmitted to the surface from various depths, but sensed conditions at a particular depth near the wellbore are generally assumed to remain substantially the same as when initially sensed. In many applications, this assumption is erroneous, and downhole sensed conditions at a selected depth change over time. In other applications, a downhole condition may not have changed, but the error rate in the transmitted signals does not provide high reliability that the sensed conditions are accurately determined. Updated sensed conditions are typically not available to the drilling operator, and accordingly most drilling operations unnecessarily incur higher risks and costs than necessary. The disadvantages of the prior art are overcome by the present invention, and an improved method and system for sensing a selected borehole or formation characteristic in a well is disclosed.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, a system for sensing a borehole or formation characteristic at a selected depth includes a plurality of sensors each positioned at axially spaced locations along a drill string, with each of the plurality of sensors sensing the selected borehole or formation characteristic. A data transmission system is utilized for transmitting signals from each of a plurality of sensors to the surface. A surface computer receives the transmitted signals and determines sensed characteristics at a selected depth as a function of one of the plurality of sensors positioned along a drill string while at a selected first depth, and subsequently another of the plurality of sensors positioned along the drill string while at substantially the first selected depth.

According to another embodiment, a method of sensing a selected borehole or formation characteristic depth includes positioning each of a plurality of sensors at axially spaced locations along a drill string, sensing the selected borehole formation characteristic with each of a plurality of sensors, transmitting signals from each of a plurality of sensors to the surface, and receiving the transmitted signals and determining a sensed characteristic at a selected depth as a function of one of the plurality of sensors positioned along the drill string while at a first selected depth, and subsequently another of the plurality of sensors positioned along the drill string while at substantially the first selected depth.

These and further features and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating one embodiment of a drill rig showing a directional drilling application and a system for sensing borehole or formation characteristics.

FIG. 2 is a functional block diagram of a suitable data transmission from the plurality of sensors.

FIG. 3 is a representative plot for analyzing measurements at the same depths for changes over time.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENT

FIG. 1 illustrates a drilling operation 10 in which a borehole 36 is being drilled beneath the surface 26 of the ground. The drilling operation includes a drilling rig 20 and a drill string 12 which extends from the rig into the borehole. A bit or other cutting device 16 is provided at the lower end of the drill string. The bottom hole assembly 15 may include a drill bit 16, a new bit sensor package 38 and a directional drilling motor or rotary steerable device 14, as shown in FIG. 1.

The drill string 12 preferably includes a plurality of network nodes 30. The nodes 30 are provided at desired intervals along the drill string. Network nodes essentially function as signal repeaters to regenerate data signals and mitigate signal attenuation as data is transmitted up and down the drill string. The nodes 30 may be integrated into an existing section of drill pipe or a downhole tool along the drill string. Sensor package 38 in the bottom hole assembly may also include a network node 30. Connectors 34 represent drill pipe joint connectors, while the connectors 32 connect a node 30 to an upper and lower drill pipe joint.

The nodes 30 comprise a portion of a downhole network 46 used to transmit information along the drill string. A downhole network may thus include multiple nodes based along the drill string. Communication links 48 may be used to connect the nodes to one another, and may comprise cables or other transmission media integrated directly into sections of the drill string. The cable may be routed through the central borehole of the drill string, or routed externally to the drill string, or mounted within a groove, slot or passageway in the drill string. Preferably signals from the plurality of sensors are transmitted to the surface through a wire conductor 48 along the drill string. Communication links may also use wireless connections.

A plurality of packets may be used to transmit information along the nodes. Packets may be used to carry data from tools or sensors located downhole to an uphole mode, or may carry information or data necessary to function the network. Other packets may be used to send control signals from the top node to tools or sensors located at various downhole positions. Further detail with respect to suitable nodes, a network, and data packets are disclosed in U.S. Publication 2005/0284663 A1 hereby incorporated by reference.

Various types of sensors may be employed along the drill string, including axially spaced resistivity, caliper, rock strength (sonic), pressure sensors, temperature sensors, seismic devices, strain gauges, inclinometers, accelerometers, bending, vibration, and rotation sensors, and flow rate sensors. Sensors which measure conditions which would logically experience significant change over time provide particularly valuable information to the drilling operator. For example, the caliper or cross-sectional configuration of a wellbore at a particular depth may change during the drilling operation due to formation stability and fluid washout conditions. The skin of a formation defining the borehole may tend to absorb fluids in the well and may thus also change over time, particularly if the well is overbalanced. By providing a system which allows a sensor to transmit to the surface at a known depth in substantially real time, a particular borehole or formation characteristic, such as the caliper of the well, and by providing another sensor which can provide the same type of information at substantially the same depth with a different sensor as the well is drilled deeper, the operator is able to compare a wellbore caliper profile at a selected depth at time one, and later measure the same caliper at substantially the same depth at time two. This allows the operator to better understand changes in the well that occur over time, and to take action which will mitigate undesirable changes. Other sensors which monitor conditions which are likely to degrade or change over time include sensors that measure wellbore stability, resistivity sensors, equivalent circulating density (ECD) measurements sensors, primary and/or secondary porosity sensors, and temperature sensors.

Other sensors may monitor conditions which are unlikely to substantially change over time, such as borehole inclination, pore pressure sensors, and other sensors measuring petrophysical properties of the formation or of the fluid in the formation. In the latter case, an operator may use the signals from different sensors at different times to make a better determination of the actual condition sensed. For example, the inclination of a wellbore at a particular depth likely will not change. The inclination measurement at time one may thus be averaged with an inclination at the same depth at time two and another inclination measurement at the same depth at time three, so that the average of these three signals at the same depth taken at three times will likely provide a more accurate indication of the actual borehole inclination.

According to the technique of the present invention, an operator at the surface may instruct a particular sensor to take a selected measurement. In most applications, however, a plurality of substantially identical sensors for sensing a particular wellbore formation characteristic will be provided along the drill string, and each of those sensors will output a signal at a selected time interval, e.g., every tenth of a second or every second, such that signals at any depth may be correlated with signals from a similar sensor at another depth. Thus an entire profile of the sensed condition based on a first sensor as a function of depth may be plotted by the computer, and a time lapse plot may be depicted for measurements from a second sensor while at the same depth at a later time. Also, it should be understood that the system may utilize sensors which are able to take reliable readings while the drill string and thus the sensors are rotating in the well, but in another application the rotation of the drill string may be briefly interrupted so that sensed conditions can be obtained from stationary sensors, then drilling resumed. In still other cases, the drill string may slide or rotate slowly in the well while the sensed conditions are monitored, with the majority of the power to the bit being provided by the downhole motor or rotary steerable device.

A significant advantage of the present invention is the ability to analyze information from the sensors when there is time lapse between a particular sensed condition at a particular depth, and the subsequent same sensed condition at the same depth. As disclosed herein, the system provides sensors for sensing characteristics at a selected depth in a well, and a particular depth may be “selected” in that the operator is particularly concerned with signals at that depth, and particularly change and rate of change for certain characteristics. Such change and rate of change (time lapse in the transmitted signals) may be displayed to the operator in real time). Otherwise stated, however, information from a sensor at selected axial locations or after a selected time lapse may be important, and the term “selected” as used herein would include a signal at any known, presumed, or selected depth.

FIG. 2 illustrates conceptually a drill pipe 12 having a plurality of axially spaced sensors 40 spaced along the drill string, each for sensing the same borehole or formation characteristic. Each sensor may be provided within one of the nodes 30 positioned along the drill string, as previously discussed. The downhole network 46 transmits information from each of a plurality of sensors to surface computer 22, which also receives information from depth sensor 50 via line 51. Depth sensor 50 monitors the length of drill string inserted in the well, and thus the output from the sensors 14 may be correlated by the computer 22 as a function of their depth in the well.

Information from the well site computer 22 may be displayed for the drilling operator on a well site screen 24. Information may also be transmitted from computer 22 to another computer 23, located at a site remote from the well, with this computer 23 allowing an individual in the office remote from the well to review the data output by the sensors 40. Although only five sensors 40 are shown in FIG. 1, those skilled in the art will understand that a larger number of sensors may be used along a drill string when drilling a fairly deep well, and that all sensors associated with any particular node may be housed within or annexed to the node 30, so that a variety of sensors rather than a single sensor will be associated with that particular node.

FIG. 3 depicts a plot of sensed borehole information characteristics numbered 1 and 2 each plotted as a function of depth, and also plotted as a function of time when the measurements are taken. For characteristic #1, pass 1 occurs first, pass 2 occurs later, and pass 3 occurs after pass 2. The area represented by 60 shows the difference in measurements between passes 1 and 2, while the area represented by 62 represents a difference in measurements between passes 2 and 3. The strong signal at depth D1 for the first pass is thus new and is further reduced for pass 2 and pass 3. For characteristic #2, the area 64 represents the difference between the pass 1 signal and the pass 2 signal, and the area 66 represents the difference between the pass 2 and pass 3 signals. For this borehole information characteristic, signal strength increases between pass 1 and 2, and further increases between pass 2 and 3.

Those skilled in the art will appreciate that various forms of markings may be employed to differentiate a first pass from a second pass, and a second pass from a subsequent pass, and that viewing the area difference under the curve of signals from different passes is only one way of determining the desired characteristic of the borehole or formation. Assuming that characteristic #2 is the borehole size, the operator may thus assume that, at a depth shortly above depth D1, the borehole has increased in size, and has again increased in size between the taking of the pass 2 measurements and the pass 3 measurements. For all of the displayed signals, signals may be displayed as a function of plurality of sensors at a single elected location in a borehole, so that a sent signal at a depth of, e.g., 1550 feet, will be compared with a similar signal from a similar sensor subsequently at a depth of 1550 feet.

Although specific embodiments of the invention have been described herein in some detail, this has been done solely for the purposes of explaining the various aspects of the invention, and is not intended to limit the scope of the invention as defined in the claims which follow. Those skilled in the art will understand that the embodiment shown and described is exemplary, and various other substitutions, alterations and modifications, including but not limited to those design alternatives specifically discussed herein, may be made in the practice of the invention without departing from its scope.

Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention. 

1. A system for sensing a selected borehole or formation characteristic at a selected depth while a drill string is in the borehole, comprising: a plurality of sensors each positioned at axially spaced locations along the drill string, each of the plurality of sensors sensing the selected borehole or formation characteristic; a data transmission system for transmitting signals from each of the plurality of sensors to the surface; and a surface computer for receiving the transmitted signals and determining the sensed characteristic at a selected depth as a function of one of the plurality of sensors positioned along the drill string while at a first selected depth and subsequently another of the plurality of sensors positioned along the drill string while at substantially the first selected depth.
 2. A system as defined in claim 1, wherein each of a plurality of sensors senses a spacing to a borehole wall at a selected depth, such that the signals from the plurality of sensors indicate a change in spacing to the borehole wall over time.
 3. A system as defined in claim 2, wherein each of a plurality of sensors measures a caliper of the borehole.
 4. A system as defined in claim 1, further comprising: a surface display for displaying signals from the plurality of sensors.
 5. A system as defined in claim 1, wherein signals from each of a plurality of sensors transmitted to the surface computer by a wire conductor spaced along the drill string.
 6. A system as defined in claim 5, wherein the transmission system includes: a plurality of nodes spaced along the drill string and interfacing with each of a plurality of sensors; and a plurality of communication links between the nodes.
 7. A system as defined in claim 1, wherein the computer distinguishes signals from the plurality of sensors as a function of their spacing along the drill string.
 8. A system as defined in claim 1, wherein the sensed characteristic are displayed at the surface as a function of depth.
 9. A system as defined in claim 1, wherein at least one of the plurality of sensors is positioned on a bottom hole assembly.
 10. A system as defined in claim 9, wherein at least one of the plurality of sensors is positioned on a bit.
 11. A method of sensing a selected borehole or formation characteristic at a selected depth while a drill string is in the borehole, comprising: positioning each of a plurality of sensors at axially spaced locations along the drill string; sensing the selected borehole or formation characteristic with each of the plurality of sensors; transmitting signals from each of the plurality of sensors to the surface; receiving the transmitted signals and determining the sensed characteristic at a selected depth as a function of one of the plurality of sensors positioned along the drill string while at a first selected depth and subsequently another of the plurality of sensors positioned along the drill string while at subsequently the first selected depth.
 12. A method as defined in claim 11, wherein each of a plurality of sensors senses a spacing to a borehole wall at a selected depth, such that the signals from the plurality of sensors indicate a change in spacing to the borehole wall over time.
 13. A method as defined in claim 12, wherein each of a plurality of sensors measures a caliper of the borehole.
 14. A method as defined in claim 11, further comprising: displaying signals from the plurality of sensors at the surface in substantially real time.
 15. A method as defined in claim 14, further comprising: displaying a rate of change in the transmitted signals.
 16. A method as defined in claim 11, wherein signals from each of the plurality of sensors are transmitted to the computer through a wire conductor along the drill string.
 17. A method as defined in claim 15, wherein a signal transmission system includes: a plurality of nodes spaced along the drill string and interfacing with each of the plurality of sensors; and a plurality of communication links between the nodes.
 18. A method as defined in claim 11, wherein a computer distinguishes signals from the plurality of sensors as a function of their spacing along the drill string.
 19. The method as defined in claim 11, wherein the sensed characteristics are displayed at the surface as a function of the depth. 